This invention relates generally to magnetic resonance spectroscopy, and more particularly, the invention relates to volume spectroscopy in which chemical shift error is reduced.
Volume localized magnetic resonance spectroscopy has become a useful and routine clinical tool especially for the detection of abnormalities which lead to diffused chemical changes in brain. Several techniques are known for directly exciting spins in a volume of interest and achieving three-dimensional selection including use of stimulated echoes and the use of Carr-Purcell echoes. These techniques obtain a localized spectrum in a single scan. For example, point resolved spectroscopy (PRESS, see U.S. Pat. No. 4,480,228) uses a three pulse sequence with each pulse being frequency selective.
Many important clinical applications of proton magnetic resonance spectroscopic imaging, MRSI, are based on phase encoding of a restricted volume of excitation. Typically, the volume excitation is achieved using PRESS, which takes advantage of three orthogonal slices in the form of a double spin echo to select a specific region of interest. Unfortunately, one weakness of this approach is that the range of chemical shift frequencies (over 200 Hz for proton at 1.5T) is not insignificant relative to the limited band width of most excitation pulses (1000-2000 Hz). The result is misregistration of the volume of interest for chemical shift frequencies not at the transmitter frequency. This is illustrated in FIG. 1. The chemical shift error causes the greatest difficulty for signals excited by the transition band portion (f) of the RF profile. Except for the portion of the pass band common to the chemical shift frequencies of interest (c), each resonance will be excited differently. Thus, when a PRESS volume is resolved by MRSI, the chemical levels are not only dependent on tissue level, T1 and T2, but are also dependent on location within the volume of interest. The only exception is within the limits of the common pass band (rf pass band(a)-chemical shift error(d)). Within the transition bands the magnitude of the error is dependent on chemical shift error and shape of the transition band. One figure of merit is the difference in excitation at the extremes of the desired chemical shift over 90% of the transition band: Max.sub.-- error=chemical shift bandwidth)* (slope of transition band). This assumes that the transition band is relatively linear and that slope represents the transition bandwidth over which excitation increases from 5% to 95% of the peak value. These values can exceed 40% for 90.degree. excitations and 60% for the refocusing pulses used in PRESS.
To reduce the uncertainty of the spatially dependent changes in the measurement of tissue levels and peak ratios in focal applications of MRSI, it is important to maximize % selectivity [defined as: (pass band/(pass band+transition bands))*100] and to minimize % chemical shift error [defined as: ((chemical shift bandwidth)/(effective rf bandwidth))*100. At typical clinical scanner B.sub.1 fields of .about.0.2 Gauss, it is difficult to design refocusing pulses as required in PRESS, with selectivity greater than 59% and chemical shift error of less than 20% for a chemical shift range of 3.4 ppm at 1.5T.
Since the selectivity in a focal MRSI application is limited by both transition band width and chemical shift, it is convenient to define the actual selectivity as: % selectivity.sub.-- mrsi=((pass band-chemical shift error)/(pass band+transition bands+chemical shift error))*100. With this definition, a typical selectivity as described above would only be 46% for a refocusing pulse and would be about 52% for a typical 90.degree. excitation. With selectivity of less than 50%, voxels resolved outside of this selectivity must either be ignored or corrected for excitation profile. Ultimately, however, it would be preferable to improve Focal MRSI selectivity, to avoid these corrections and concomitant assumptions.
Le Roux et al., U.S. Pat. No. 5,537,039 for VIRTUAL FREQUENCY ENCODING OF ACQUIRED NMR IMAGE DATA addresses this problem in effect by distorting the phase encoded frequencies to match the slice selection misregistration. This is accomplished by adding evolution time encoding to each phase encode increment. The downside for spectroscopic imaging are the impacts of evolution time encoding on coupled spins, and the impact of exciting large signals such as lipid outside the volume of interest.